1. Field of the Invention
This invention is directed to novel coated superparamagnetic alloy nanoparticles and methods to prepare such materials. In particular, the invention is directed to silica coated iron cobalt ternary alloy nanoparticles having a metal silicate layer interfacing the metal alloy and the silica coating.
2. Discussion of the Background
Iron cobalt alloys are conventionally utilized in the construction of magnetic cores of motors, generators and transformers. Conventionally, such cores have been constructed of laminate structures of magnetic alloys, typically iron-cobalt-vanadium or iron-cobalt chromium alloys. Such laminate structures generally consist of alloy metal layers sandwiched with interlaminar insulation and binder layers. These interlaminar layers are necessary to insure high electrical efficiency of the magnetic core.
However, ever increasing demand for greater and more efficient performance of motors, generators and transformers has spurred a search for new materials with which compact magnetic cores having greatest saturation induction and little or no hysteresis loss can be constructed.
The most important characteristics of such soft magnetic core components are their maximum induction, magnetic permeability, and core loss characteristics. When a magnetic material is exposed to a rapidly varying magnetic field, a resultant energy loss in the core material occurs. These core losses are commonly divided into two principle contributing phenomena: hysteresis and eddy current losses. Hysteresis loss results from the expenditure of energy to overcome the retained magnetic forces within the core component. Eddy current losses are brought about by the production of induced currents in the core component due to the changing flux caused by alternating current (AC) conditions.
The use of powdered magnetic materials allows the manufacture of magnetic parts having a wide variety of shapes and sizes. Conventionally, however, these materials made from consolidated powdered magnetic materials have been limited to being used in applications involving direct currents. Direct current applications, unlike alternating current applications, do not require that the magnetic particles be insulated from one another in order to reduce eddy currents.
Conventionally, magnetic particles are coated with thermoplastic materials to act as a barrier between the particles to reduce induced eddy current losses. However, in addition to the relatively high cost of such coatings, the plastic has poor mechanical strength and as a result, parts made using plastic-coated particles have relatively low mechanical strength. Additionally, many of these plastic-coated powders require a high level of binder when pressed. This results in decreased density of the pressed core part and, consequently, a decrease in magnetic permeability and lower induction. Additionally, and significantly, such plastic coatings typically degrade at temperatures of 150-200° C. Accordingly, thermoplastic coated magnetic particles are of limited utility.
Conventionally, ferromagnetic powders have been employed for the production of soft magnetic core devices. Such powders are generally in a size range measured in microns and are obtained by a mechanical milling diminution of a bulk material. Superparamagnetic nanoparticle materials having particle size of less than 100 nm have found utility for magnetic record imaging, as probes for medical imaging and have been applied for targeted delivery of therapeutic agents. However, these utilities have generally been limited to superparamagnetic iron oxide nanoparticles and little effort has been directed to the development of iron-cobalt ternary alloy nanoparticles suitable for utilization in the production of core magnetic parts.
Brunner (U.S. Pat. No. 7,532,099) describes coated alloy particles which are employed with a ferromagnetic alloy powder and a thermoplastic or duroplastic polymer to prepare an injection molded or cast soft magnetic core. An alloy of Iron, copper, niobium, silicon and boron is heat treated to form a nanocrystalline structure, then comminuted in a mill to obtain particles having dimensions of about 0.01 to 1.0 mm. An abrasion resistant layer of iron and silicon oxide of 150 to 400nm is coated to the particles.
Anand et al. (U.S. Pat. No. 6,808,807) describes encapsulated ferromagnetic powders obtained by coating a ferromagnetic core with a polyorganosiloxane or polyorganosilane and thermally treating the coated core to convert the polymer to a residue containing silicon and oxygen. The core alloy may be any of iron alloyed with silicon, aluminum, nickel, cobalt, boron, phosphorous, zirconium, neodymium and carbon. Ferromagnetic core particles having an average diameter of less than 2 mm are suitable for this composition.
Gay et al. (U.S. Pat. No. 6,193,903) describes ceramic coated ferromagnetic powders. The powders are iron or an iron alloy and the encapsulating layer on the particle may be one of a group of ceramics such as a metal oxide, metal nitride, metal silicate and a metal phosphate. The particle size is from 5 to 1000 microns. Silica is listed as one of a large group of ceramic materials suitable as the coating.
Moorhead et al. (U.S. Pat. No. 6,051,324) describes particles of an alloy of iron/cobalt/vanadium having a particle size of less than 44 microns which are coated with a glass, a ceramic or a ceramic glass, including silicon dioxide.
Atarashi et al. (U.S. Pat. No. 5,763.085) describes a magnetic particle having multiple layers on its surface which is useful as a starting material for color magnetic materials such as magnetic toners. The particles are of a size from 0.01 to 200 μm. Silicon dioxide is described as a metal oxide coating along with preparation by a sol gel method. Description of preparation of a metal layer on the particle by reduction of a soluble metal salt in the presence of a complexing agent is provided.
Yamanaka et al. (U.S. Pat. No. 4,983,231) describes a surface treated magnetic powder obtained by treating an iron-rare earth metal alloy with alkali-modified silica particles. The mean particle diameter of the alloy particles is from 20 to 200 μm. Upon heating the alkali silicate dehydrates and condenses to form a “polysiloxane” coating.
Uozumi et al. (JP 2007-123703) describes application of a silicate coating to soft magnetic powders including alloys of iron, cobalt and vanadium, having a mean particle size of 70 microns. The coated particles are heat treated cause migration of Si and O into the soft magnetic core to form a diffusion zone between the outer oxide layer and the soft magnetic core.
Yamada et al. (JP 03-153838) (Abstract) describes surface treatment of an iron/cobalt/vanadium powder with a compound containing silicon and an alkoxy group (such as vinyl triethoxysilane). No description of particle size or method to produce the alloy particle is provided.
Sun et al. (J. Am. Chem. Soc., 2002, 124, 8204-8205) describes a method to produce monodisperse magnetite nanoparticles which can be employed as seeds to grow larger nanoparticles of up to 20 nm in size.
Bumb et al. (Nanotechnology, 19, 2008, 335601) describes synthesis of superparamagnetic iron oxide nanoparticles of 10-40 nm encapsulated in a silica coating layer of approximately 2 nm. Utility in power transformers is referenced, but no description of preparation of core structures is provided.
Zhang et al. (Nanotechnology, 19, 2008, 085601) describes synthesis of silica coated iron oxide particles. The average size of the iron oxide particle to be coated is 8 to 10 nm and the silica core is about 2 nm.
Hattori et al. (U.S. 2006/0283290) describe silica coated, nitrided iron particles having an average particle diameter of 5 to 25 nm. The particles are “substantially spherical” and are useful for magnetic layers such as a magnetic recording medium.
Yu et al. (J. Phys. Chem. C 2009. 113, 537-543) describes the preparation of magnetic iron oxide nanoparticles encapsulated in a silica shell. Utility of the particles as magnetic binding agents for proteins is studied.
Thus, there is a need for new magnetic powders to produce soft magnetic parts, which provide increased green strength, high temperature tolerance, and good mechanical properties and which parts have minimal or essentially no core loss.
Previous effort in this study is described in prior U.S. application Ser. No. 13/558,397, filed Jul. 26, 2012, the disclosure of which is incorporated herein by reference in its entirety. Application to magnetic cores is described in prior U.S. application Ser. No. 13/565,250, filed Aug. 2, 2012, the disclosure of which is incorporated herein by reference in its entirety.
However, there is a need for new and/or improved magnetic powders to produce soft magnetic parts, which provide increased green strength, high temperature tolerance, and good mechanical properties and which parts have minimal or essentially no core loss.
Therefore, an object of the present invention is to provide a superparamagnetic powder which has tunable magnetic properties suitable to produce soft magnetic parts, while simultaneously having increased green strength, high temperature tolerance, and good mechanical properties for the production of high performance magnetic cores.
A second object of the invention is to provide a method to prepare the powder nanoparticles of such superparamagnetic powder.